EchoLink is an awesome bit of software that allows you to do a form of Radio-Over-IP (RoIP). That means you’re talking to other stations over the internet, anywhere in the world. Whether from your laptop in a coffee shop, or from your smartphone as you drive during your commute, you can call CQ and make your QSLs across the world. One thing of note is that while it is available worldwide, it’s only open to the Amateur Radio community as it requires a valid call sign to create an account.
Currently, EchoLink is only available on Windows 7 and above (link). However, it is available on App Store for Apple devices and the Google Play store for Android devices.